This invention relates to an optical automatic levelling apparatus for use in leveling the ground setting-out cooperation with a light receptor by emitting a laser beam in a horizontal direction of from 0.degree. to 360.degree. about a vertical axis to form a horizontal plane.
A heretofore known optical automatic levelling apparatus has a construction such as is shown in FIG. 9. A support plate c is fixed onto four support poles b which are mounted on a mount bed a. The mount bed a is kept horizontal by three leveling screws i. A laser diode d is fitted to the lower surface of the support plate c facing downwardly. A convex lens f is suspended by three suspension wires e, the opposite ends of which are fixed to the support place c in such a manner that even when the apparatus is inclined within a range of about 10', the convex lens f always moves in the direction of gravitational force by its own weight and is positioned in a perpendicular direction of the laser diode d. A rotary penta mirror h (which consists of mirrors m.sub.1 and m.sub.2) which reflects the parallel laser beams g leaving the convex lens f in the orthogonal direction thereto, that is, in the horizontal direction, is rotatably supported on the mount bed a. Furthermore, the apparatus includes a motor (not shown) for rotating the rotary penta mirror h and a damper (not shown) for preventing vibration of the convex lens f to prevent a measurement error due to the vibration of the convex lens at the time of inclination of the apparatus.
In accordance with the conventional apparatus described above, the support poles b exist around the outer periphery of the rotary penta mirror.Accordingly, when any support pole b exists between the penta mirror h and the light receptor (not shown), the ray of light emitted from the rotary penta mirror h is intercepted by the support pole b and does not reach the light receptor. Therefore, the conventional apparatus involves the inherent problem that it includes four positions where measurement cannot be made.
The checking operation of the vibration of the convex lens by the damper described above (hereinafter called an "automatic compensation action") can be improved by increasing the length of the suspension wires e. However, when a convex lens having a large focal length is used at the same distance as when the suspension wire e is short, the angular aperture .theta. (the angle of expansion of the rays of light of the light source incident on the convex lens f) becomes small so that the quantity of light of the laser beam passing through the convex lens f becomes small and eventually, the reach of the laser beam becomes short and the range of measurement becomes narrow. On the other hand, when a convex lens f having the same angular aperture as when the suspension wire e is short is used and the range of the measurement is the same, the convex lens f is great in size and heavy in weight so that the reliability of automatic compensation action drops. Generally, the automatic compensation action cannot be improved as a whole.
For the reasons described above, the conventional apparatus involves the problem that the effect of the automatic compensation action cannot be improved without making the range of measurement narrow.
The present invention is directed to provide an optical automatic levelling apparatus which solves the provlems of the prior art apparatuses described above.
In an optional automatic levelling apparatus including a light source, a lens means for converting the ray of light emitted from the light source to a parallel ray of light perpendicular to a true horizontal plane, a rotary reflector capable of rotation, for reflecting the parallel ray of light from the lens means in a horizontal direction and a driving motor for rotating the rotary reflector, the optical measuring apparatus to accomplish the object described above is characterized in that the lens means consists of a concave lens and a convex lens, the light source is disposed on a mount bed in such a manner as to face upwardly, the concave lens is suspended from a support member extending from the mount bed by suspension wires, the convex lens is disposed above the concave lens by the support member and the rotary reflector is disposed at the upperpart of the support member.
In this specification, a true horizontal plane is one which is perpendicular to an imaginary vertical line between the center of the earth and a point in consideration.
The length of the suspension wires for suspending the convex lens is set to ##EQU1## so that even when the apparatus is inclined as a whole at a minute angle such as within the range of about 10', the rays of light incident to the rotary reflector become always perpendicular-to-horizontal rays of light. Here, d.sub.1 is the distance between the concave lens and the convex lens, d.sub.2 is the distance between the light source and the concave lens and f.sub.2 is the focal length of the concave lens. The reason why this relationship is established will be explained next.
Assuming that the apparatus is inclined as a whole at a minute angle with a point p.sub.1 on the support member as the center as shown in FIG. 1, the light source moves from a point O.sub.1 to a point O.sub.2 while the center c.sub.1 of the convex lens moves to c.sub.2. The center of the concave lens would move from a poinst c.sub.3 to a point c.sub.4 at the time of inclination if fixedly supported, but actually remains unchanged because it returns to its original position in accordance with the principle of a pendulum because it is supported by the suspension wires.
In FIG. 1, it will be assumed that the rays of light of the light source leaving the point O.sub.2 turn to light ray l.sub.1 through the concave lens and the convex lens (through the lens center c.sub.2).
In order for this ray of light l.sub.1 to become the perpendicular-to-horizontal light ray (the ray of light parallel to the perpendicular axis O.sub.1 p.sub.1), the ray of light l.sub.3 of the light source leaving the point O.sub.2 must be refracted by the convex lens, becomes the perpendicular-to-horizontal ray of light l.sub.2 and pass through the center c.sub.2 of the convex lens. To satisfy this condition, the ray of light l.sub.3 must pass through the reflection point c.sub.3 ' of the concave lens and be incident on the focus of the concave lens.
The condition necessary for satisfying the relationship described above are as follows:
In FIG. 2 which is an expanded view of the state shown in FIG. 1, when the rays of light l.sub.1 and l.sub.2 are parallel to the perpendicular-to-horizontal axis O.sub.1 p.sub.1, EQU .theta..multidot.L.sub.2 .apprxeq.y.sub.1 +y.sub.2 EQU Therefore, EQU .theta..multidot.L.sub.2 .apprxeq..theta..multidot.d.sub.1 +.alpha..multidot.f.sub.2 ( 1)
Since .alpha.=.theta.+.beta. and .theta..multidot.d.sub.1 =.beta..multidot.d.sub.2 (with the proviso that .theta..apprxeq.0, .beta..apprxeq.0, .alpha..apprxeq.0), the formula (1) becomes as follows: ##EQU2##
The distance L.sub.2 between this point p.sub.1 and the concave lens is the length of the suspension wire.
As obvious from the formula (2), the length of the suspension wire can be elongated while keeping constant the angular aperture of the lens, that is, the distance d.sub.2 between the concave lens and the light source, by changing the distance d.sub.1 between both lenses and the focal length f.sub.2 of the concave lens.
In order to convert the ray of light of the light source leaving the point O.sub.1 of to the parallel rays of light through the concave lens and the convex lens, the distance L.sub.1 between the light source and the convex lens, the distance d.sub.2 between the light source and the concave lens, the distance d.sub.1 between both lenses, the focal length f.sub.1 of the convex lens and the focal length f.sub.2 of the concave lens are determined in such a manner as to satisfy the following relationship while the relationship described above is satisfied: ##EQU3## with the proviso that in the formula (3) and (4), F represents the focal length of the synthetic lens system consisting of the convex lens and the concave lens.
In FIG. 3, O.sub.1 p.sub.1 represents the perpendicular-to-horizontal line passing through the center C.sub.3 of the concave lens and the center c.sub.1 of the convex lens.
In the manner described above, a concave lens which is lighter in weight than the convex lens can be suspended from the support member through long suspension wires so that the automatic compensation action can be improved without making the range of measurement narrow. The perpendicular-to-horizontal ray of light leaving the convex lens is reflected by the rotary reflector disposed rotatably at the upper part of the support member in an orthogonal direction, that is, in the horizontal direction, and the true horizontal rays of light can be emitted in all directions without being intercepted by any support member.
Hereinafter, an embodiment of the present invention will be explained with reference to the accompanying drawings.